More on the srcset proposal

After a lot of reading over the weekend and a fair bit of discussion on the WHATWG mailing list,  I’ve come to a couple of conclusions:

  1. Initially, I totally misunderstood how <img srcset> works
  2. I still don’t want it, and still do want <picture>

The appeal of <picture> is that you can select different image sources based on any media query, giving you fine control over which version is served under various conditions. Currently this includes viewport width/height, device orientation, pixel density, colour capability etc, and as more media queries arrive (surely there will be a connection speed query sooner or later, and I’d love to see containing element width/height as well) the element becomes even more useful and flexible. Initially, I thought <img srcset> was an attempt to avoid creating a new element by shoehorning this functionality into a new attribute.

In fact, <img srcset> comes from a radically different angle. For each version of the image, we add an entry to the srcset attribute with the filename, and optionally the width, height and resolution (expressed as “1x”, “2x” etc). The user agent then decides which version to serve, based on the metadata we’ve given it and the current conditions on the client (viewport, connection speed, pixel density, user settings, etc). Unlike <picture>, authors aren’t specifying which source to use and when; rather, we’re just telling the user agent what’s available and letting it make the call on its own. <img srcset> allows authors to provide lots of different versions of an image, but affords us no control over when they are used.

Update: I had this wrong, and so did quite a few other people. There’s still some ambiguity, with the spec contradicting itself in places. Jeremy Keith has done an excellent job of clearing up what the parameters in srcset actually do in his post today.

When we practise responsive design, we are trying to give the best possible experience to our users by adapting our websites to work with whatever they are using to access them. <picture>, by design, gives us the control to do this with content images. <img srcset>, by design, takes it away.

Of course, <picture> is imperfect (duplication of alt attributes is annoying, as is the potential duplication of media queries on some pages), but in my view it’s the best path to continue down.

16 Responses to More on the srcset proposal

  1. David Clements says:

    This depends on how well user agents handle the srcset.

    If widths, heights and pixel ratios are optional how does the user agent determine the correct image?

    I would like to see browsers implement some version of both concepts, srcset serves those who want some responsiveness but less work – and also terse code.

    Picture serves those who require finer grain control over how their site responds to different scenarios and are willing to chew on the verbosity.

    srcset could be adapted to give more control with some sort of explicit prioritization flag

    • David Goss says:

      The suggested logic for choosing the source is described here:
      http://junkyard.damowmow.com/507

      I suspect that in reality different user agents would have different views on the best way to choose. Not saying that’s a bad thing, but it would make the results unpredictable.

      • David Clements says:

        Ah I see, regarding this “those that are wider than the available width”

        How is that determined?

        Say if your screen is 1024px, and you have an image element set to 25%, is available width therefore 256px?

        • David Goss says:

          That’s unclear, but I have to assume so.

          It’s interesting, because on the WHATWG mailing list the other day I mentioned how nice it would be to have a media query that tests containing element width, to which the response was effectively “you can’t wait until after the CSS is applied to decide which image to download”. Okay, but it’s hard to see how the srcset proposal would work properly without waiting for CSS to be applied so the “available width” is known.

          • David Clements says:

            In his long email, Ian Hickson addresses the “available width”. He basically says change it from available width to viewport (e.g. window.width) because you can’t get the available width without processing layout information.

            “The net result of this [processing srcset before getting layout info] is that we can just change the proposal above to
            use the viewport dimensions instead of the available width, and it should
            work about as well. It does mean, though, that we can’t use the height=””
            and width=”” attributes as fallback dimensions for the other ones.”

            I don’t really see how this is going to work… if I have 3 images, one at 256, one at 512 one at 1024, and my viewport is 1024, but the image is 25%, by this setting the image according to view port width the UA is going to load the 1024 image, which is four times larger than what’s required

          • David Goss says:

            Indeed.

            <picture> could work better here because authors know the context the image is going to be in and can specify appropriate versions accordingly.

            Because of the shortcoming you describe, I feel I could only use srcset for handling different pixel densities – a valid and increasingly important use case, but only one part of the responsive images problem in my view.

  2. Kornel says:

    Giving control to authors means it works ONLY in cases predicted by the author and will not work with uncommon/future cases.

    Days of IE6 are gone. Now we have high-quality UAs that are updated more often than all pages on the Internet :)

    And connection speed media query is an awful awful idea. I don’t want images cached on WiFi to be discarded and redownloaded in lowres when I move to mobile connection.

    • Jacob Mather says:

      Your complaint about cached images sounds like a browser configuration setting to me.

      Media queries and responsive design are all about working with things we can’t predict and future cases.

    • David Goss says:

      As it happens, I do share your concerns about a connection speed media query. There’s also the opposite scenario to what you describe: a normal quality version gets cached whilst on a slow connection, then you move to a fast connection – does the UA serve the normal quality version from the cache, or download a high quality version?

      Anyway, there would hopefully be some logic in place to prevent the kind of unwanted behaviour you’re talking about.

  3. Ariel says:

    I fear that if you try to do too much this entire proposal will collapse.

    Do just one thing, and do it well: Different images for different DPI’s of the agent.

    You want a different image for different screen sizes? That’s something else, and should not be mixed in because you will get bogged down getting the details perfect, technology will march on and someone will come up with an imperfect solution that’s good enough, and then this entire working group will be for naught.

    • David Goss says:

      This group was not formed to find a solution just for different pixel densities. The point was, and is, to be able to serve different sources based on a variety of conditions.

      I would argue that by reusing the syntax of <video> and also reusing media queries, the <picture> proposal is a very uncomplicated solution. It’s not “trying to do too much”, it’s trying to do exactly what we want.

      • David says:

        If this group was not formed to find a solution for pixel densities, srcset is not a concern for you. You are solving completely different problems. srcset’s purpose is to handle higher resolution versions of the same image. picture is to select different images depending on what layout. Whether srcset is used to solve the pixel density problem is unrelated to whether picture is used to solve your problem. (Except in so much as the syntaxes interacting. Presumably the source element should get a srcset attribute too.)

        On displays with high pixel densities, CSS pixels don’t correspond to device pixels. They get scaled up so each CSS pixel is, e.g., 2×2 device pixels. It’s as if you view the page with a 200% zoom. Correspondingly, images are “scaled up” so that one pixel on the image is one CSS pixel. This is not ideal for high pixel densities. Ideally, on an image where CSS pixels are scaled double for device pixels, we could get an image sized in device pixels and drawn “scaled down” in CSS pixels so that, at the end of the day, pixels in the image correspond to pixels in the screen. The usual way to do that is to combine an image with more pixels with some way to scale the image.

        srcset solves that problem. In fact, picture doesn’t even address that use! You need not only to select the larger image, but also to specify that the image is scaled down in CSS pixels. Picture does not provide the latter. Picture also cannot address this as, fundamentally, this is a UA decision. Likewise, srcset fundamentally cannot solve your problem as this is an author decision. Nor has it ever tried to. This conflict would be easily resolved if the grop stopped viewing it as an “us versus them” thing. It’s not. You have been comparing apples and oranges.

  4. Ariel says:

    Also, by using existing css media options, you can do the same thing already, by using display: none;

    In my experience browsers don’t download the image if it’s display none, so there is no bandwidth issue. I read the FAQ about CSS and it doesn’t make sense.

    http://mislav.uniqpath.com/2010/04/targeted-css/

    Use that, put a display: none on all the images except one, and done. I don’t get why you need a new picture tag.

    • David Goss says:

      Incorrect. UAs will still download an <img> even if it is display:none in CSS. <img> downloads begin very early in the life of the page, before any CSS is applied (even inline styles).

      See question three:
      http://www.w3.org/community/respimg/common-questions-and-concerns/

  5. Pingback: Responsive images: what's the problem, and how do we fix it? | Vutphala.com

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